Significance of the genetic relationships deduced from partial nucleotide sequencing of infectious bursal disease virus genome segments A or B.

The rapid genomic characterization of infectious bursal disease virus (IBDV) requires determining which partial nucleotide (nt) sequences derived from IBDV segments A or B would produce phylogenetic information as significant as sequencing the whole corresponding segments. Long nt coding sequences of 27 IBDV segments A (aa 20-991) and 21 segments B (aa 7-stop codon) were retrieved from databanks and used to compute reference phylogenetic trees using Neighbor Joining (NJ) and Parsimony (P): clusters appearing in the NJ and P reference trees with a bootstrap value greater than 80% were considered as significant (Whole Segment Clusters, WSC). The sequences were then cut into overlapping regions. These were used to compute phylogenetic trees which were compared with reference ones. Of the partial sequences, the VP2 gene best represented IBDV segment A (10 out of 13 WSC were conserved), and the 5' two thirds of segment B best represented segment B (5 to 6 conserved WSC out of 6). Implementation of the Plato programme finally demonstrated that the region encoding VP2 variable domain (vVP2, segment A) is the only region of IBDV genome with a significantly different evolution rate, which result is consistent with vVP2 being subjected to a high selection pressure.